You still get what's going on, but removing the element of sound (even if that sound is nothing more than additional lines and colors of ink on paper just like the character illustrations) greatly diminishes the impact of the action. In part this has to do with how our minds process comics panels.

Sound exists only in time. It is inherently dynamic, unable to exist as a static phenomenon. And yet comics are inherently static, still images on a printed page existing as-is regardless of how much (if any) time passes around them. Combining those two diametrically opposed elements greatly enhances comics' ability to trick our minds into believing that we are watching action, that what appears on the page is unfolding in time.

OnomontoPOWia is not unique in this ability, of course, but I believe it is one of the most powerful due to the unique nature of sound. When you see "THWAP!" you can't help but imagine you are hearing it out loud, carrying the images associated with it forward as well. Like a movie without a soundtrack, the medium of comics can still be effective without onomontoPOWia, but it's a much different experience.